Metallic sheet metal parts are frequently coated with a layer of plastic for protection, appearance, and as an alternative to painting. For sheet metal parts having a consistent cross-sectional shape along the length, such as extruded or roll-formed parts, the plastic is typically extruded over a sheet metal substrate. In the extrusion process, the preformed sheet metal substrate is pushed or pulled through an extrusion die simultaneously with a molten thermoplastic material so that a laminated composite strip is produced.
Generally, the laminating of a plastic material onto the sheet metal substrate in an extrusion process has been carried out only after the sheet metal substrate has been formed to its final shape. According to this prior art technique, it is difficult to control the dimensional tolerances of the final laminated product because the sheet metal substrate is preformed to a contoured shape. Also, re-tooling is expensive because for each different cross-sectional shape, a separate extrusion die must be manufactured, at appreciable cost.
The prior art has partially solved this problem by laminating the substrate with plastic prior to contouring the substrate. That is, the plastic is laminated to the substrate while still in the flat, unformed, or raw, condition. One example of this prior art technique is shown in U.S. Pat. No. 4,578,858 to Adell, issued Apr. 1, 1986. The primary advantage derived from extruding the laminate onto flat substrate is that the extrusion die may remain unchanged while the final shape contour of the finished product varies.
However, even the prior art extrusion dies for extruding onto flat unformed substrates are cumbersome to use and cannot be precisely controlled to vary the thickness of the extruded plastic lamination layer. Further, the lamination process is slow because only one sheet metal substrate is moved through the extrusion die at a time.